In an electronic component, a high programming potential is generally necessary, to program a storage cell for example. This high programming potential HV is greater than the normal potential VDD that can be supported by the component and is generally produced by a potential generator internal to the component, for example a charge pump type circuit which produces the high potential HV from the normal potential VDD. The presence of a high-potential generator is used especially by the user of the component to program the memory cells of the component by himself and hence parameterize the component alone.
However, charge pump type circuits may have low fan-out, i.e., they produce limited current at the output (the power of the circuit is naturally limited by the power given to the component), and the cells are therefore programmed one after the other; this programming sequence may be relatively lengthy, and may even be prohibitive when a large number of components are programmed, for example during the component-testing stages at the manufacturer's plant.
To overcome this difficulty, manufacturers generally plan for an additional power pin on the component, to enable direct power supply at high potential by an external source that is more powerful and more efficient than a charge pump type of internal circuit. Powering the component by means of an external source enables the parallel programming of a large number of storage cells. It then becomes feasible to program a large number of components in a reasonable amount of time.
If the component has both an external power supply pin and an internal high-potential power generator, a selector switch device is used for the selection of either the internal or the external high-potential power source.
FIG. 1 shows a prior-art selector switch device capable of supporting and switching over the high potentials applied to its terminal. The circuit of FIG. 1 has two arms each having n series-connected transistors, M0, M2, . . . , M2n−2 for one arm and M1, M3, . . . , M2n−1 for the other arm. The first arm is connected between a first input to which a potential HV0 is applied and an output of the selector switch device, and the second arm is connected between a second input to which a potential HV1 is applied and the output of the selector switch device at which HVout is produced.
Two bias circuits 10, 20 (also called “cascode-bias circuits”) are used to control respectively the transistors of the first arm and the transistors of the second arm. The circuits 10, 20 are activated by external control signals. HV0 is for example a high potential given by an external source and HV1 is for example a high potential given by an internal source of the component. The selector switch device of FIG. 1 gives a potential HVout at its output. This potential HVout is equal either to the potential HV0 or to the potential HV1 depending on the external control signals.
In each arm, the transistors are transistors capable of taking a maximum voltage across their electrodes that is approximately equal to the nominal potential VDD of the component. The number n of transistors is chosen as a function of the potentials HV0, HV1 so as to distribute the voltage along the unselected arm of the selector switch device. The number n is thus of the order of (HV1—HV0)NDD in terms of absolute value. Further details on the making of the selector switch device of FIG. 1 are found in “A 5.5V SOPA Line Driver in a Standard 1.2V 0.13 μm CMOS Technology”, Proceedings of ESSCIRC, Grenoble, France, 2005, which is incorporated by reference).
The device of FIG. 1 may have drawbacks. In particular, the device of FIG. 1 is efficient for switching over the potentials HV1, HV0 up to about 2*VDD, but it is no longer efficient if HV1, HV0 are greater than 2*VDD, because the bias circuits typically must switch over large numbers of cascade-connected transistors. Furthermore, the bias circuits 10, 20 may be difficult to make because they use complex circuits such as potential step-up circuits. Finally, the bias circuits must be controlled by external control signals and additional means may be needed to produce these signals.